# safe_eval
# Copyrighted (C) Michael Spencer
#
# Source: http://code.activestate.com/recipes/364469/

import compiler

class Unsafe_Source_Error(Exception):
    def __init__(self,error,descr = None,node = None):
	self.error = error
	self.descr = descr
	self.node = node
	self.lineno = getattr(node,"lineno",None)

    def __repr__(self):
	return "Line %d.  %s: %s" % (self.lineno, self.error, self.descr)
    __str__ = __repr__

class SafeEval(object):

    def visit(self, node,**kw):
	cls = node.__class__
	meth = getattr(self,'visit'+cls.__name__,self.default)
	return meth(node, **kw)

    def default(self, node, **kw):
	for child in node.getChildNodes():
	    return self.visit(child, **kw)

    visitExpression = default

    def visitConst(self, node, **kw):
	return node.value

    def visitDict(self, node,**kw):
	return dict([(self.visit(k),self.visit(v)) for k,v in node.items])

    def visitTuple(self, node, **kw):
	return tuple(self.visit(i) for i in node.nodes)

    def visitList(self, node, **kw):
	return [self.visit(i) for i in node.nodes]

    def visitUnarySub(self, node, **kw):
	return -self.visit(node.expr)

class SafeEvalWithErrors(SafeEval):

    def default(self, node, **kw):
	raise Unsafe_Source_Error("Unsupported source construct",
				node.__class__,node)

    def visitName(self,node, **kw):
	raise Unsafe_Source_Error("Strings must be quoted",
				 node.name, node)

    # Add more specific errors if desired


def safe_eval(source, fail_on_error = True):
    walker = fail_on_error and SafeEvalWithErrors() or SafeEval()
    try:
	ast = compiler.parse(source,"eval")
    except SyntaxError, err:
	raise Unsafe_Source_Error(err, source)
    try:
	return walker.visit(ast)
    except Unsafe_Source_Error, err:
	raise